This invention relates to doppler speed measuring system incorporating a digital adaptive filter device and more particularly to a ship-mounted speed measuring device where the device is capable of providing the velocity of the ship with an acceptable error under high rates of acceleration and also capable of providing smooth velocity data when the ship has a steady state velocity.
Doppler speed logs or measuring devices provide processed individual returns from a sonar signal which have considerable scatter in the computed velocity from pulse to pulse. There are a number of reasons why the computed or measured velocity has substantial variation from return to return. One reason is the finite beam width of the usual sonar speed measuring device which typically will provide a plus or minus 7.5 percent doppler spread. In addition, motions of the ship on which the doppler speed measuring device is mounted cause motions of the transducer (i.e. roll in athwartship direction, and pitch in the longitudinal direction). In addition, when the sonar speed detection relies upon the signal reflected from the water mass as when the ship is in deep water, there is an additional component of velocity spread produced by the motion of the water itself. Lastly, there is ambient noise which, in practice, produces the least amount of velocity uncertainty since operation is generally at high signal-to-noise ratios.
In order to smooth the data received from each sonar transmission, the data from a number of consecutive valid readings of the doppler speed were averaged prior to being provided as an output of the speed measuring device. For speed updates occurring approximately every five seconds and for repetition rates which would be low enough to avoid multiple signals being returned from reflecting surfaces during the interpulse interval, the number of pulse returns which were averaged would typically be nine or thirty-six returns. The average signal would also be subjected to fixed slew rate limits of 0.1, 0.2, 0.3, or 0.4 knots for every five second update. Thus, the velocity of the ship presented at the output of the doppler measuring device could not vary more than the slew rate limit from output to output at the five second rate. Virtually all ships used the 0.1 knot velocity change for the maximum velocity change per five second upward velocity update. For large commercial vessels which had low accelerations, this low slew rate of 0.1 knots per second was acceptable to adequately follow accelerations and to provide speed data that was substantially constant for constant speed operation.
For those ship installations where the acceleration of the ship could not be followed by a 0.1 knot per second velocity update at the five second update rate, the doppler speed measuring devices were found to be lacking in the presentation of accurate speed information under high acceleration rates even though the velocity readings under steady state conditions were acceptable. To remedy this deficiency, the system cycle update rate was increased to substantially one update per second thereby reducing the number of sonar doppler returns which were averaged by a factor of four to provide nine returns per cycle thereby causing the raw data being processed by the doppler speed measuring device to have substantially greater variation in doppler frequency on each update. A binary adaptive filter was added to allow the speed measuring device to track accelerations up to 20 knots per minute with acceptable lag. The binary adaptive filter had the capacity for providing four slew rates of 0.1, 0.2, 0.4, and 0.8 knots per update.
The binary adaptive filter operated in the following manners. If it is assumed that the past speed reading was V.sub.N-1 knots and the gate width was G.sub.N-1 knots per update, the first step would be to subtract the past speed reading, V.sub.N-1, from the present reading, V.sub.N, thereby providing an output .DELTA..sub.N difference speed in knots. If .DELTA..sub.N exceeded G.sub.N-1, then the output would be V.sub.N-1 +G.sub.N-1 if .DELTA..sub.N was positive, and V.sub.N-1 -G.sub.N-1 if .DELTA..sub.N was negative. In either case, since .DELTA..sub.N exceeded G.sub.N-1, the filter would increase the value of G.sub.N-1 to the next highest allowed value for the processing of the next return signal. Alternatively, if .DELTA..sub.N was less than G.sub.N-1, the present speed reading V.sub.N would be outputted and the gate value G.sub.N-1 would be changed to the next smaller increment. If .DELTA..sub.N =0, the output speed would be unchanged and the gate G.sub.N would be reduced by one increment. This selection process of providing an updated speed output and changing of the gate value would continue for each cycle of the speed measuring system.
A filter built to follow the preceding rules of operation attempts to reduce the slew rate down to the lowest value of 0.1 knots per update during constant speed running. When an acceleration or deceleration occurs, the filter opens up the gate width in order to attempt to track the velocity change with an acceptable error. However, the scatter in the raw velocity readings provided to the filter even at constant speed running following acceleration or deceleration results in keeping the gates open to a value greater than desired. Thus, the digital speed display shows excessive variation in velocity on each update. As a result, users of the speed measuring device using the filter just described find its presentation to be unacceptable because the displayed velocity perturbations run counter to the expected constant velocity of the ship which is cruising with constant engine rpm's. FIG. 1, plot 20, shows the output speed variation as a function of time of this prior art filter for a simulated signal provided in the laboratory in which an FM oscillator was provided to the doppler system which simulated a speed of 10 knots with a plus or minus 2 knot variation at a one second rate. The plots 21, 22 is the output speed when simulating acceleration/deceleration, respectively, from 10 to 20 knots in one minute with a plus or minus 2 knot speed variation. The speed output plots 23, 24 were obtained with an acceleration/deceleration, respectively, of 10 to 20 knots in one minute with no variation in speed during the acceleration period. It is noted that even under cruising conditions when the speed should be constant at 10 knots, the output speed readings have undesirable fluctuations of about plus or minus 0.5 knot relative to the 10 knot average speed.